Automatic gain control



July 27, 1937.

" G. Joss-r AUTOMATIC GAIN CONTROL Filed Aug. 17, 1955 games tl? 'IIIIIII INVENTOR 60A/THE@ `M657 BY Wg ATTORNEY Patented July 27, 1937 "-UN1TED sra'iss PArENT OFFICE f I 2,088,206 l AUTOMATIC GAIN CONTROL Gunther Jobst, Berlin, Germany, assigner to Telefunken Gesellschaft lfr Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation of Germany Application August 17, 1933, Serial No. 685,533 In Germany September 3, 1932 5 claims. (ci. 25o-2,0)

In radio receivers circuit schemes are known which insure compensation of the fading of the incoming signals by that the working point of one, or several, amplier tubes on the non-linear part ofthe characteristic is shifted as a function of the signal strength in such a way that ythegain is diminished whenever the signal istrength isrlarger. By schemes of the said sort it isessentially the slope of the regulated tubes that is altered.

11' Contradistinct from the prior art as outlined, f circuit arrangements comprising screen grid 'tubes of special design are disclosed in the present invention in which the internal resistance of r the tube `(which under certain circumstances may be negative) may even be reversed in sign so that in this manner regulable amplication is insured. The advantage residing in this novel method is that the range of regulation is eX- panded several times seeing that, according to the size of the internal resistance either reduction of damping or increased damping is occasioned in the useful circuit.

'I'he novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one arrangement whereby my invention may be c-arried into effect.

Referring to the drawing, the screen grid tube which is used according to this invention is characterized by six electrodes, the relative position and action of which is indicated in the single gure. The tube I contains a heated cathode 2, .a control grid 3, a screen grid 4 and an apertured plate 5. In -addition to these electrodes contained in the standard type of screen grid tube, there are provided a so-called baiile grid 6 which is negatively biased by the total bias from the series arranged voltage sources 6 and I5 as shown in the drawing, and anotherrplate, or anode, 1 connected with a positive potential.

The outstanding feature of this tube is that the internal resistance is altered as a function of the control grid voltage. In fact, the same may not only be caused to assume zero value (be compensated), but may even assume negative values or become overcompensated. In this latter case the damping of the plate impedance associatedV With the tube is reduced. The dependence of the internal resistance is such that by the increase of the negativeV control-grid voltage the plate current is diminished, and that upon a reduction of the current the value of the negative internal resistance is decreased so that the damping reduction of the associated circuit becomes less. The change in the internal resistance incidentally is in the main ascribable to the influence of the baffle grid 6. However, there arises at the same time, as is well known, also a variation in slope and of the gain-reciprocal acting in the same sense. i

Now, this circumstance is profit-ably utilized for the compensation of fading effects in that the circuit scheme, by ways and means basically well known in the art, is so chosen that whenever large radio frequency amplitudes arise there is rendered operative a higher negative control grid bias. As

`a result, in the presence of larger amplitudes and higher grid bias the value of the negative internal resistance of the tube is reduced, and this means less damping reduction with the consequence that the gain drops. Where lower input voltages and lower grid biasing voltages are concerned the value of the negative internal resistance grows so that the damping reduction and the amplication or gain `are raised. By choosing a suitable inter-relationship between the change in internal resistance and the biasing voltage acting at the control grid it is feasible to obtain extensive compensation of the fading effects.

The part of a receiver outfit which has a bearing on this invention is illustrated in a fundamental and rgeneral way in the figure. 'Ihe input voltage is impressed upon the control grid 3 of tube I by way of the coils 8 and 9 and the control grid 3 ebeing negatively biased by voltage source I5 as shown in the drawing. The coil 9 may, of course, be tuned to the incoming signal by any well known means. The amplified oscillations are fed by way of the tuned circuit Ill, II, to the grid circuit coil I2 of audion, or demodulator, tube I3. The plate circuit of tube I3 contains an audio transformer III- in series with a resistance I5 which at the same time forms part of the control grid circuit of tube I. A blocking condenser I6 shunts, or bridges, the resistance I5, and constitutes a short-circuit path for radio frequency energy. The operation of the regulator device here disclosed resides in that, in the presence .of strong signals a lesser fall of potential is occasioned across the resistance I5 whereby the negative bias of the control grid 3 is increased by virtue of the negative bias from source I5.

Another mode to alter the internal resistance of tube I resides in influencing of the negative biasing voltage of the baiile grid 6, or else a simultaneous utilization of the grids 3 and 6.

What is claimed is:

1. A signal transmission system comprising a tube provided with an electron emission element, a signal grid, and a positive anode, a positive cold electrode disposed between the grid and anode and at a greater positive potential than the latter, a negative auxiliary grid disposed between the anode and positive electrode, a signal input circuit connected between the signal grid and emission element, a signal output circuit connected between the positive electrode and the emission element, and means responsive to signal amplitude variations for automatically adjusting the direct current potential difference between the emission element and at least one of said grids.

2. A signal transmission system comprising a tube provided with an electron emission element, a signal grid, and a positive anode, a positive cold electrode disposed between the grid and anode, a negative auxiliary grid disposed between the anode and positive electrode, a signal input circuit connected 'between the signal grid and emission element, a signal output circuit connected between the positive electrode and the emission element, and means, including a demodulator tube having an impedance in its space current path connected in said signal input circuit, responsive to signal amplitude variations for automatically adjusting the direct current potential difference between at least the signal grid and the emission element.

3. An automatic control arrangement for a signal amplier tube, the tube including a cathode, signal grid, a positive perforated plate, a screen grid between the signal grid and plate, a negative baille grid and a positive plate at a lesser positive potential than the perforated plate, the baffle grid being disposed between the two plates, a signal input circuit between the signal grid and cathode, a signal output circuit between the perforated plate and the cathode, a signal demodulator tube adapted to produce a direct current control voltage from signals, and means for impressing the control voltage between the cathode and at leastone of the grids for adjusting the internal resistance of the amplifier tube.

4. In an arrangement as defined in claim 3, said last means comprising a resistor in the said signal input circuit, the resistor being disposed in the demodulator tube space current path.

5. A signal transmission system comprising an electron discharge tube provided with a cathode, a signal input grid, a positive output electrode, a positive anode at a direct current potential which is less than the potential of said output electrode, and a negative baffle grid disposed between the anode and said output electrode, said tube having a characteristic such that its negative internal resistance is decreased upon a reduction of its output circuit current, a signal rectifier having an input circuit coupled to the output electrode circuit of said tube, a resistor included in the space current path of said rectier, and connections between the input electrodes of said tube and said resistor such that the gain of said tube is decreased upon an increase in received signal amplitude.

'GNTHER J OBST. 

